linux-kernel - Re: [PATCH v2 8/8] powerpc: Enable Generic Entry/Exit for syscalls.

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Message-ID: <065e3421-0ae1-4e1b-8384-2157d2c1daf9@kernel.org>
Date: Tue, 16 Dec 2025 07:41:11 +0100
From: "Christophe Leroy (CS GROUP)" <chleroy@...nel.org>
To: Mukesh Kumar Chaurasiya <mkchauras@...ux.ibm.com>, maddy@...ux.ibm.com,
 mpe@...erman.id.au, npiggin@...il.com, oleg@...hat.com, kees@...nel.org,
 luto@...capital.net, wad@...omium.org, mchauras@...ux.ibm.com,
 thuth@...hat.com, sshegde@...ux.ibm.com, charlie@...osinc.com,
 macro@...am.me.uk, akpm@...ux-foundation.org, ldv@...ace.io, deller@....de,
 ankur.a.arora@...cle.com, segher@...nel.crashing.org, tglx@...utronix.de,
 thomas.weissschuh@...utronix.de, peterz@...radead.org,
 menglong8.dong@...il.com, bigeasy@...utronix.de, namcao@...utronix.de,
 kan.liang@...ux.intel.com, mingo@...nel.org, atrajeev@...ux.vnet.ibm.com,
 mark.barnett@....com, linuxppc-dev@...ts.ozlabs.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 8/8] powerpc: Enable Generic Entry/Exit for syscalls.



Le 14/12/2025 à 14:02, Mukesh Kumar Chaurasiya a écrit :
> From: Mukesh Kumar Chaurasiya <mchauras@...ux.ibm.com>
> 
> Convert the PowerPC syscall entry and exit paths to use the generic
> entry/exit framework by selecting GENERIC_ENTRY and integrating with
> the common syscall handling routines.
> 
> This change transitions PowerPC away from its custom syscall entry and
> exit code to use the generic helpers such as:
>   - syscall_enter_from_user_mode()
>   - syscall_exit_to_user_mode()
> 
> As part of this migration:
>   - The architecture now selects GENERIC_ENTRY in Kconfig.
>   - Old tracing, seccomp, and audit handling in ptrace.c is removed in
>     favor of generic entry infrastructure.
>   - interrupt.c and syscall.c are simplified to delegate context
>     management and user exit handling to the generic entry path.
>   - The new pt_regs field `exit_flags` introduced earlier is now used
>     to carry per-syscall exit state flags (e.g. _TIF_RESTOREALL).
> 
> This aligns PowerPC with the common entry code used by other
> architectures and reduces duplicated logic around syscall tracing,
> context tracking, and signal handling.
> 
> The performance benchmarks from perf bench basic syscall are below:
> 
> perf bench syscall usec/op
> 
> | Test            | With Patch | Without Patch | % Change |
> | --------------- | ---------- | ------------- | -------- |
> | getppid usec/op | 0.207795   | 0.210373      | -1.22%   |
> | getpgid usec/op | 0.206282   | 0.211676      | -2.55%   |
> | fork usec/op    | 833.986    | 814.809       | +2.35%   |
> | execve usec/op  | 360.939    | 365.168       | -1.16%   |
> 
> perf bench syscall ops/sec
> 
> | Test            | With Patch | Without Patch | % Change |
> | --------------- | ---------- | ------------- | -------- |
> | getppid ops/sec | 48,12,433  | 47,53,459     | +1.24%   |
> | getpgid ops/sec | 48,47,744  | 47,24,192     | +2.61%   |
> | fork ops/sec    | 1,199      | 1,227         | -2.28%   |
> | execve ops/sec  | 2,770      | 2,738         | +1.16%   |

I get about 2% degradation on powerpc 8xx, and it is quite stable over 
time when repeating the test.

'perf bench syscall all' on powerpc 8xx (usec per op):

| Test            | With Patch | Without Patch | % Change |
| --------------- | ---------- | ------------- | -------- |
| getppid usec/op | 2.63       | 2.63          | ~ 0%     |
| getpgid usec/op | 2.26       | 2.22          | +2,80%   |
| fork usec/op    | 15300      | 15000         | +2,00%   |
| execve usec/op  | 45700      | 45200         | +1.10%   |

Christophe